Liquid nitrogen, which has a boiling point at about 77K, is commonly employed in the industry, hospitals, and biomedical laboratories as the main cryogenic material to provide a low-temperature environment. However, even with heavy insulations, some heat, albeit very small, will gradually penetrate into the liquid nitrogen container, which can be in the form of a tank or wall linings, and cause the liquid nitrogen to vaporize. In order to ensure that the cryogenic environment is always maintained at or below a predetermined temperature, the amount of the liquid nitrogen in the container must be constantly monitored, and refilled if it falls below a predetermined level.
Conventionally, the level of the liquid nitrogen is monitored using a monitoring device belonging to the electric-capacitor type. The conventional monitoring device utilizes a pair of coaxial stainless steel tubes; it is operated based on the principle that liquid and gaseous nitrogen exhibits different electric capacitances, and a non-zero differential capacitance will be recorded if the upper tube contains gaseous nitrogen. One of the shortcomings of the conventional type monitoring/control system for liquid nitrogen refill is that it often malfunctions due to its relatively low sensitivity and thus fails to provide the intended control function, in addition to its relatively high manufacturing cost. As a result, the liquid nitrogen level is typically monitored and refilled manually with human labor. However, manual operation has it own undesirable aspects due to the inevitable human errors such that the liquid nitrogen often became completely vaporized before it was detected, causing irreparable damages to the test specimen, failure in experiments and/or loss in production.